1.
Segment V10.1: Bicycle Through a Puddle
(Related to Textbook Section 10.2.2 - Froude Number Effects)
The wave pattern made by an object moving through water depends on whether the object moves slower or faster than the wave speed. These conditions correspond to a Froude number (Fr) less than or greater than one.
Riding a bike through a puddle produces surface waves. If the bike is stationary (Fr=0), the waves spread out as concentric circles. A slowly moving bike (Fr<1) generates waves that travel outward in all directions, although relative to the bike they move more slowly upstream than downstream. At higher speeds when the bike travels faster than the wave (Fr>1) a characteristic V-shaped wave pattern is produced.
2.
Segment V10.2: Merging Channels
(Related to Textbook Section 10.4.1 - Uniform Flow Approximations)
The simplest type of open channel flow is one for which the channel cross-sectional size and shape and the water depth remain constant along the length of the channel. In many situations these conditions are not met.
Although the flow in each of the two merging channels of the model study shown may be essentially uniform flow upstream of their confluence, the actual merging of the two streams is quite complex and far from uniform flow. How the two streams mix to produce a single stream may be of considerable importance to downstream locations. (Video courtesy of U.S. Army Engineer Waterways Experiment Station.)
3.
Segment V10.3: Uniform Channel Flow
(Related to Textbook Section 10.4.2 - The Chezy and Manning Equations)
The Manning Equation is used to determine the constant-depth flowrate in a straight channel with constant slope and constant cross-section.
Although the geometry for many man-made channels is sufficiently uniform to allow the use of the Manning equation, the irregularity of many channels (especially natural ones) makes the use of the Manning equation a rough approximation at best. Curves in the channel, variable flowrate along the channel (as with rainwater runoff into a gutter), or irregular channel cross-section may cause the calculated flowrates to be quite different than the actual flowrate.
4.
Segment V10.4: Erosion in a Channel
(Related to Textbook Section 10.6 - Rapidly Varied Flow)
In many situations it is the complex two- or three-dimensional flow in an open channel that is of interest (as opposed to uniform channel flow). Such flows are often very difficult to analyze theoretically.
The complex three-dimensional flow structure at the end of the floor of the channel in this model test is responsible for the severe erosion along the channel bed. Without an appropriate design to eliminate the undercutting of the channel floor, the entire channel bottom would soon be washed downstream. (Video courtesy of U.S. Army Engineer Waterways Experiment Station.)
5.
Segment V10.5: Hydraulic Jump in a River
(Related to Textbook Section 10.6.1 - The Hydraulic Jump)
If the liquid velocity in an open channel is fast enough and depth shallow enough, a hydraulic jump (a step-like increase in liquid depth) may occur. For this to happen, the Froude number must be greater than one. The flow must be supercritical.
Shown here is the flow exiting from under a dam. The Froude number of the flow is greater than one, and a large hydraulic jump occurs. Significant energy is dissipated within the turbulent, agitated flow across the hydraulic jump.
6.
Segment V10.6: Hydraulic Jump in a Sink
(Related to Textbook Section 10.6.1 - The Hydraulic Jump)
The hydraulic jump is a step-like increase in water depth across which a supercritical flow can change into a subcritical flow.
It is possible to generate a hydraulic jump of circular shape on a plate held under the faucet in the sink. The location and size of the hydraulic jump depend on the flowrate and distance from the faucet to the plate. Near the center of the plate the thin, highspeed flow is supercritical. The lip on the plate edge forces the flow to be relatively deep and low-speed (subcritical) at the edge. The hydraulic jump provides the adjustment between these subcritical and supercritical flows.
7.
Segment V10.7: Triangular Weir
(Related to Textbook Section 10.6.2 - Sharp-Crested Weirs)
A triangular weir can be used to measure flowrate in a channel.
A thin vertical plate with a V-notch cut into it is placed across the channel and the flowrate is determined by measuring the depth (head) of the water above the bottom of the notch. Flowrate from an irrigation tank into experimental agricultural plots is determined by the use of weirs. The sensitivity of the flowrate to the water depth is a function of the angle of the V-notch of the weir. The smaller the angle, the greater the head needed for a given flowrate.
8.
Segment V10.8: Low-Head Dam
(Related to Textbook Section 10.6.2 - Sharp-Crested Weirs)
Flow over a sharp-crested weir or a low-head dam can be quite complex and dangerous, especially if the flow contains a plunging nappe.
As shown in a model experiment involving flow over a low-head dam, it is possible to develop a "roller" on the down stream side of the dam from which it is difficult to dislodge floating objects. Items caught in this eddy tend to remain there. An appropriate change in the downstream geometry can be made to eliminate the hazards associated with the original design. (Video courtesy of U.S. Army Engineer Waterways Experiment Station.)